The present invention pertains generally to a joint prosthesis. In particular, the present invention relates to absorbing forces in a joint prosthesis.
The human body includes several joints such as the knee, the hip, the shoulder and the elbow. These joints are vulnerable to damage from various injuries, wear and disease. When the joints have been severely damaged, partial or total joint replacement may be the only viable solution. In many joint replacements, a prosthetic structure is inserted into the joint. Typically, the prosthetics include a base member secured to a bone to allow normal joint articulation.
The human knee is the single largest joint of the human body, but due to its structure, is arguably the most vulnerable to damage. The leg consists principally of a lower bone called a tibia and an upper bone known as the femur. The tibia and femur are hinged together at the knee joint. The knee joint includes several femoral condyles supported in an engagement with crescentic fibrocartilages that are positioned on the upper end of the tibia and receive the femur. The joint is held together by numerous ligaments, muscles and tendons. The patella is a similarly supported bone positioned in front of the knee joint and acts as a shield for it.
In addition to providing mobility, the knee plays a major role in supporting the body during static and dynamic activities. The knee works in conjunction with the hip and ankle to support the body weight during static erect posture. The knee is also heavily loaded because of its location connecting the two longest bones in the human body. Body weight, inertia and ground reaction forces often produce large moments at the knee. Dynamically, the knee joint must transmit extremely high forces needed for powerful movement of the lower extremity, while damping out impulsive shock loads to the spine and head. Furthermore, the knee must provide major stability to the lower extremity as well as fulfill major mobility roles during movement.
In current knee replacement prosthetic designs, the tibia is resected to form a flat, horizontal platform known as a tibial plateau. A tibial platform is secured to the tibial plateau with posts or anchors fixed normal or perpendicular to the tibia plateau. The anchors provide additional support to the tibial platform when the joint is subjected to shear, tipping and torque forces present under normal knee articulation.
A similar component, comprising a curved convex semi-spherical shell, covers the femoral condyles and slidably engages a concave tibial bearing insert. On a side opposite the femoral component, the tibial insert is substantially flat and slidably engages the tibial platform. Interaction of opposing surfaces of these three elements, the femoral component, the tibial component, the tibial insert and the tibial platform allows the prostheses to function in a manner equivalent to a natural knee joint.
Current prosthetic designs are relatively inflexible and inelastic, especially when reacting to forces produced on the knee joint. When a prosthesis is placed in-vivo, the prosthesis experiences a larger number of force cycles that can ultimately lead to failure of the prosthesis. As a result, a prosthesis is needed that can absorb and limit failure over a larger number of force cycles.